1. Industrial Field of the Invention
The present invention relates to a pressure sensor of a piezoelectric type suitable for detecting a pressure such as a combustion pressure within a cylinder of an internal combustion engine.
2. Description of the Related Art
Conventionally, there has been often employed a piezoelectric-type pressure sensor which utilizes a piezoelectric effect that application of stress induces generation of electric change. Especially in recent years, development of a pressure sensor suitable for detecting a combustion pressure within a cylinder of an internal combustion engine or the like has been prosperous. FIG. 5 illustrates a basic structure of a piezoelectric-type pressure sensor disclosed in Japanese Patent Unexamined Publication No. 63-109342. This is a vertical cross-sectional view of the sensor in which a piezoelectric element 34 is provided at the inside of a sensor casing 31. When an upper fixing screw 35 is tightened through a screw portion 35a, the piezoelectric element 34 is pressed through a pressure transmitting member 33 and fixed against the backside of a pressure receiving surface 32 machined in the form of a diaphragm, while it is given pre-stress. This pre-stress is required for measuring a negative pressure particularly when measuring a combustion pressure within a cylinder of an internal combustion engine.
The operation of the pressure sensor will now be described.
A pressure to be measured is applied to the pressure receiving surface 32 from the outside of the sensor casing 31. This pressure is transmitted through the pressure transmitting member 33 so as to press an inner peripheral portion of the cylindrically-shaped piezoelectric element 34 while an upper outer-peripheral portion of the piezoelectric element 34 is depressed by reaction of the upper fixing screw 35. As a result, a shear force is given to the piezoelectric element 34 because the downward farce is exerted on its upper outer-periphery whereas the upward force is exerted on the lower inner-periphery. Then, there is generated an electric charge corresponding to this stress which is detected in the form of an electric signal from electrodes (not shown) provided on the piezoelectric element 34.
However, the above-described arrangement involves the following problems.
A first problem will be explained referring to FIG. 6. In order to fit the pressure sensor onto an engine body 36 to be subjected to pressure measurement, a sensor casing thread portion 31a is fitted onto a threaded bore in the engine body 36, and then, the sensor casing 31 is tightened into the engine body 36. As the sensor casing 31 is tightened, the lower portion of the sensor casing 31 which includes the thread portion 31a is pressed in a direction indicated by an arrow A while the upper portion of the sensor casing 31 is retained by the top surface of the engine body 36 and affected by a force in a direction indicated by arrows B. Regarding the sensor casing 31 as a whole, therefore, tensile stress is generated with respect to a portion in the vicinity of a seal member 40, resulting in distortion in its axial direction. Under such a situation, the position of the upper fixing screw 35 is fixed. However, as the sensor casing thread portion 31a is tightened, the position of the pressure receiving surface 32 and the position of the pressure transmitting member 33 are displaced in the direction A, and pre-stress which is preset relative to the piezoelectric element 34 is gradually decreased, thereby generating a fluctuation of the sensor output unfavorably. Besides, the contact of the piezoelectric element 34 with the pressure transmitting member 33 and with the upper fixing screw 35 becomes unstable to make the transmitted pressure non-uniform or to make the pressure transmission impossible.
A second problem will now be explained. The pressure receiving surface 32 of the sensor casing 31 is usually pressed in the axial direction by a tightening force of the upper fixing screw 35 through the pressure transmitting member 33 and the piezoelectric element 34. A portion where the pressure receiving surface 32 is in contact with the pressure transmitting member 33 is illustrated in an enlarged cross-sectional view of FIG. 7. Referring to this drawing, due to the tightening force of the upper fixing screw, the pressure receiving surface 32 is pressed toward the side opposite to the pressure transmitting member 33 and bent downwardly. Then, since the pressure transmitting member 33 is formed of a ceramic material or the like having high rigidity which transmits the pressure effectively, the pressure receiving surface 32 thus bents is not in surface-contact with the pressure transmitting member 33, but only an outer peripheral portion of the end face of the pressure transmitting member 33 is in contact with the backside of the pressure receiving surface 32. As a result, the stress concentrates on this contact portion alone, and when the pressure receiving surface 32 is affected by a force from a region 38 to be measured, the outer peripheral portion of the end face of the pressure transmitting member 33 is cracked by the concentrated stress so that the pressure transmitting member 33 thus broken unfavorably interferes with accurate measurement of the pressure.